Phase 3 (Treatment Period): Eligible patients will be enrolled in a two-armed 6-week period of double-blind placebo-controlled acute treatment. All subjects who continue to meet eligibility criteria will be randomized to one of two groups: GSK561679 (at a fixed dose of 350 mg/day) or placebo. Randomization will be performed at a 1:1 ratio into two treatment groups. Neuropsychological and neurophysiological testing will be repeated after 5 weeks of the double-blind treatment period.